Synthetic oil base stocks having viscosities of about 4 to 30 cSt or above at 100.degree. C. have been prepared in prior art by oligomerisation of olefins by conventional or modified Friedel-Craft catalysts. Thus by contacting the alphaolefins with boron trifluoride containing various promoters, synthetic oils suitable for lubricant use have been prepared, such as described in U.S. Pat. Nos. 4,400,565; 5,068,487 and 5,191,140. However, boron trifluoride being pulmonary irritant is fast being replaced by less hazardous catalysts like aluminum halides.
A number of aluminum halide catalyst systems have been disclosed for oligomerisation of alpha olefins to poly alpha olefins which could be used as lubricating oil base stocks possessing low pour points, higher viscosity index and good oxidation stability. U.S. Pat. No. 3,637,503 discloses the oligomerisation of alpha olefins having from 4 to 16 carbon atoms in the presence of aluminum chloride and a non-polymerizing hydrocarbon diluent. Similarly aluminum chloride alone or along with organic promoters have been used to oligomerise alpha olefins either pure or in presence of a non-oligomerising hydrocarbon diluent. See for example U.S. Pat. Nos. 5,196,635; 5,136,118; 4,107,080; 4,219,691 and 4,031,159. It is also known in the related field the possibility of oligomerising olefins in which the double bond is statistically distributed along the entire carbon chains. Thus U.S. Pat. No. 4,167,534 discloses tile oligomerisation of olefins obtained from a PACOL-OLEX process, by contacting with aluminum chloride to obtain oligomers which after distillation and catalytic hydrogenation gave the lubricating oil. Even though the feed stock for oligomerisation was predominantly olefinic (up to 95%), the yield and the viscosity of the resulting oligomer were very poor.
However, all these processes pertain to oligomerisation of either pure or mixtures of pure alpha-olefins, and suprisingly there are no reports on the utilisation of linear olefins contained in refinery streams, for the production of synthetic lubricant base stocks. Various refinery produced cracked distillate streams, particularly from Coker and FCC units, are quite rich in desired alpha-olefins, which can be selectively concentrated by the process of urea adduction.
A major deficiency with the conventional Friedel-Craft catalysts is their inability to selectively oligomerise olefins in the presence of other unsaturated compounds like aromatics. However, aluminum chloride alone or with promoters is known to promote the alkylation of aromatics with olefins, when applied to cracked refinery distillate streams which contained appreciable amounts of aromatics alongwith olefins. The oligomeric product thus obtained contain alkylated aromatics, which made these products unsuitable for use as lubricating oils, because of very poor oxidation stability. No prior art method either discloses or teaches any catalyst system which can selectively oligomerise olefins to poly olefins in the presence of aromatics. Consequently, it is infact impossible to prepare an olefin oligomer having high viscosity index and high oxidation stability which could be qualified for such uses as gas turbine oil, hydraulic fluid for air crafts, crankcase oils, etc by selective oligomerisation of olefins present in the cracked refinery stream distillates which also contain aromatics, besides paraffins and cycloparaffins, by use of Friedel-Craft catalyst systems disclosed in the prior art. The present invention provides for a catalyst composition for preparation of olefin oligomers suitable for use as lubricating oils by selective oligomerisation of olefins, contained in cracked refinery stream distillates, which have been processed through a step of urea adduction.